The operation of a fuel injection system is well known in the art. Air enters the engine through the air induction system where it is measured by an air flow meter. As the air flows into the cylinder, fuel is mixed into the air by the fuel injector. Fuel injectors are arranged in the intake manifold behind each intake valve. The injectors are typically electrical solenoids that are operated by the engine control unit, or “ECU.” The ECU pulse the injectors by switching the injector ground circuit on and off to release fuel into the cylinders.
When the injector is turned on, the valve opens to spray atomized fuel at the back side of the intake valve. As fuel is sprayed into the intake airstream, it mixes with the incoming air and vaporizes. The ECU preferably causes the injector to deliver fuel to achieve an ideal air/fuel ratio for the given engine, e.g. 14.7:1 for many engines, often referred to as the stoichiometry. The precise amount of fuel delivered to the engine is a governed by ECU control. That is, the ECU determines the basic injection quantity based upon the measured intake air volume and engine speed. Depending on other engine operating conditions, injection quantity will vary. The ECU can monitor variables such as coolant temperature, engine speed, throttle angle, and exhaust oxygen content to make injection corrections that determine final injection quantity.
In order to achieve the precise balance of fuel and air, the most important characteristic in the determination is the cylinder air mass. There are a number of methods to calculate cylinder air volume in order to correctly meter injected fuel. The most common method is to measure air mass flow using an air flow meter (AFM), also known as a mass air flow (MAF) sensor, which measures the air flow into the engine. With the air flow (in mass units per time) and the engine speed, the air mass in each cylinder can be calculated by dividing the air mass flow over the engine speed, number of cylinders, and number of engine rotations per induction strokes.
Once the cylinder air mass is known, calculating the fuel injector open time to achieve a desired air-fuel ratio requires calculating the mass of fuel required (from the cylinder air mass and desired air-fuel ratio) and then looking up a reference table for the fuel injectors to determine the injector opening time to deliver the fuel mass. A common expression for the actual air-fuel ratio over the stoichiometric optimal ratio is referred to as “lambda.” The preceding two steps may be combined into one lookup table (from cylinder air mass to injector duration), or may be separated into tables indexed by fuel pressure, fuel temperature, battery voltage and other variables.
Air flow meters use a variety of methods to measure air flow, including hot wire and Karmen Vortex. These AFMs require a turbulence free air flow past the sensor in order to correctly meter the air flow. However, engine induction air flow consists of pulsations from each cylinder induction event. After-market engine modifications typically including larger duration camshafts, forced induction, high flow exhaust headers, etc., that can increase the pulsation effect in the intake system of the engine to such an extent that the AFM can no longer correctly meter the incoming air (see FIG. 1). Additional problems arise if the engine intake system is altered, which can cause the air flow past the AFM to be different from that which the ECU was calibrated for (see FIG. 2). This leads to greater scatter in the data and reduced performance.